Loose powders

Examples are provided by the work of Carman and Raal with CF2CI2 on silica powder, of Zwietering" with nitrogen on silica spherules and of Kiselev" with hexane on carbon black and more recently of Gregg and Langford with nitrogen on alumina spherules compacted at a series of pressures. In all cases, a well defined Type II isotherm obtained with the loose powder became an equally well defined Type IV isotherm with the compact moreover both branches of the hysteresis loop were situated (drove the isotherm for the uncompacted powder, but the pre-hysteresis region was scarcely affected (cf. Fig. 3.4). The results of all these and similar... 

Abrasive appHcations for industrial diamonds include thek use in rock drilling, as tools for dressing and tmeing abrasive wheels, in polishing and cutting operations (as a loose powder), and as abrasive grits in bonded wheels and coated abrasive products. 

Apparent Density. This term refers to the weight of a unit volume of loose powder, usually expressed in g/cm (l )- The apparent density of a powder depends on the friction conditions between the powder particles, which are a function of the relative surface area of the particles and the surface conditions. It depends, furthermore, on the packing arrangement of the particles, which depends on the particle size, but mainly on particle size distribution and the shape of the particles. 

The characteristics of a powder that determine its apparent density are rather complex, but some general statements with respect to powder variables and their effect on the density of the loose powder can be made. (/) The smaller the particles, the greater the specific surface area of the powder. This increases the friction between the particles and lowers the apparent density but enhances the rate of sintering. (2) Powders having very irregular-shaped particles are usually characterized by a lower apparent density than more regular or spherical ones. This is shown in Table 4 for three different types of copper powders having identical particle size distribution but different particle shape. These data illustrate the decisive influence of particle shape on apparent density. (J) In any mixture of coarse and fine powder particles, an optimum mixture results in maximum apparent density. This optimum mixture is reached when the fine particles fill the voids between the coarse particles. 

Tap Density. Tapping a mass of loose powder, or more specifically, the appHcation of vibration to the powder mass, separates the powder particles intermittently, and thus overcomes friction. This short-time lowering of friction results in an improved powder packing between particles and in a higher apparent density of the powder mass. Tap density is always higher than apparent density. The amount of increase from apparent to tap density depends mainly on particle size and shape (see Table 4). 

Porous parts and bearings are made by both the press and sinter techniques, whereas filters are made by loose powder sintering. The metals most commonly used for P/M porous products are bron2e, stainless steel (type 316), nickel-base alloys (Monel, Inconel, nickel), titanium, and aluminum. 

The basic manufacturing process involves thorough blending of the components, especially the pigments, and comminution with the aid of a variety of mills to reduce the particle size. Loose powders ate filled without additional processing. 

Sample Preparation Pressed Disc of Mull Loose Powder... 

Diffuse Reflectance IR Fourier Transform Spectroscopy (DRIFTS) can be employed with high surface area catalytic samples that are not sufficiently transparent to be studied in transmission. In this technique, the diffusely scattered IR radiation from a sample is collected, refocussed, and analysed. Samples can be measured in the form of loose powders. 

A very powerful method for the evaluation of solubility differences between polymorphs or solvates is that of intrinsic dissolution, which entails measurements of the rates of solution. One method for this work is to simply pour loose powder into a dissolution vessel, and to monitor the concentration of dissolved solute as a function of time. However, data obtained by this method are not readily interpretable unless they are corrected by factors relating to the surface area or particle size distribution of the powder. In the other approach, the material to be studied is filled into the cavity of a circular dissolution die, compressed until it exhibits the effective planar surface area of the circular disc, and then the dissolution rate is monitored off the surface of the rotating disc in the die [130],... 

The types of intrinsic dissolution profiles obtainable through the loose powder and constant surface area methods are shown in Fig. 19. Oxy-phenbutazone was obtained as the crystalline anhydrate and monohydrate forms, with the monohydrate being the less soluble [129]. The loose powder dissolution profiles consisted of sharp initial increases, which gradually leveled off as the equilibrium solubility was reached. In the absence of supporting information, the solubility difference between the two species cannot be adequately understood until equilibrium solubility conditions are reached. In addition, the shape of the data curves is not amenable to quantitative mathematical manipulation. The advantage of the constant surface area method is evident in that its dissolution profiles are linear with time, and more easily compared. Additional information about the relative surface areas or particle size distributions of the two materials is not required, since these differences were eliminated when the analyte disc was prepared. 

Fig. 19 (a) Loose powder aqueous dissolution profiles for the solubilization at 37°C... 

Van Santen [13] identifies three levels of research in catalysis. The macroscopic level is the world of reaction engineering, test reactors and catalyst beds. Questions concerning the catalyst deal with such aspects as activity per unit volume, mechanical strength and whether it should be used in the form of extrudates, spheres or loose powders. The mesoscopic level comprises kinetic studies, activity per unit surface area, and the relationship between the composition and structure of a catalyst and its... 

In the diffuse reflectance mode, samples can be measured as loose powders, with the advantages that not only is the tedious preparation of wafers unnecessary but also diffusion limitations associated with tightly pressed samples are avoided. Diffuse reflectance is also the indicated technique for strongly scattering or absorbing particles. The often-used acronyms DRIFT or DRIFTS stand for diffuse reflectance infrared Fourier transform spectroscopy. The diffusely scattered radiation is collected by an ellipsoidal mirror and focussed on the detector. The infrared absorption spectrum is described the Kubelka-Munk function ... 

The angle of repose may also be measured using a plane sheet to which is stuck a layer of particles from the powder. Loose powder is then poured on to the sheet which is then tilted until the powder slides. The angle of slide is known as the static angle of repose or the drained angle. 

Two important morphological parameters characterizing ball-milled powders are the particle and grain size of constituent phases within the powders. In our laboratory, the size measurement of the powder particles is carried out by attaching loose powder to sticky carbon tape and taking pictures under secondary electron (SE) mode in the SEM. The images are then analyzed by an image analysis software. The size of the powders is calculated as the particle equivalent circle diameter, ECD = AA/nf, where A represents the projected particle area. Usually from -300 to 700 particles are analyzed for each batch. 

At first this new assumption seemed confirmed by the enhanced activities of catalysts which were obtained by pressing loose powders of iron oxide into dense tablets. Soon, however, it was found that magnetites of another origin than the Swedish specimen yielded no ammonia, or only negligible amounts, in spite of the dense structure of their reduction products. 

The purpose of a delay composition is obvious - to provide a time delay between ignition and the delivery of the main effect. Crude delays can be made from loose powder, but a compressed column is capable of much more reproducible performance. The burning rates of delay mixtures range from very fast (millimeters/millisec ond) to slow (millimeters /second). 

A slower-burning mixture is preferred over a more rapid one. The slower release of energy allows for better heat transfer to the main composition. Also, most "first fires" are pressed into place or added as moist pastes (that harden on drying), rather than used as faster-burning loose powders. 

This behavior is typical of loose powders, and points out the potential danger of confining mixtures that burn quite sluggishly in the open air. 

A loose powder should burn the fastest, perhaps reaching an... 

Laminae Binder (Laminae 99 Lupersol 1%) NC lacquer or celluloid binder can also be used as a loose powder... 

Figure 2.31 Development of ceramic microstructure during sintering (a) Loose powder particles (b) initial stage (c) intermediate stage and (d) final stage. From W. E. Lee and W. M. Rainforth, Ceramic Microstructures, p. 37. Copyright 1994 by William E. Lee and W. Mark Rainforth, with kind permission of Kluwer Academic Publishers.

Modern Manufacturing Techniques. Manufacturing techniques for making bulk vitreous silica are for the most part improved variations of the historical processes. The main exception is the sol—gel process (see Sol-gel technology). All processes involve the fusion or viscous sintering of silica particles. The particles can be in the form of a loose powder or a porous preform. The powders can be made from natural quartz or from the decomposition of chemical precursors, such as silicon tetrachloride, and tetraethylorthosilicate (1 EOS). In some approaches, such as flame hydrolysis, the powder is produced and fused in a single step. The improvements made to these techniques deal mainly with the procedures used to prepare the powders, that is, to control purity and particle size, and the specific conditions under which the powders are consolidated. 

Moreover, the formation of copper trihydroxychlorides accelerates the corrosion of the remaining metal, and is accompanied by an important volume expansion, which results in a fragmentation of the object. This process known as bronze disease can be identified by the formation of spots or patches of a light green loose powder on the surface of the object (Fig. 5.5). 

Pyrotechnics are invariably intimate mixtures of finely powdered fuels and oxidizers together with other additives in order to impart special effects depending on the requirement. Pyrotechnic formulations burn very rapidly when ignited as a loose powder. However, there are some applications where controlled steady burning is required for a definite time and this is achieved by compressing the mixture into columns surrounded by a relatively non-combustible material so that combustion proceeds from one end to other, layer by layer for a long duration. 

Thermal decomposition of y-lactone tosylhydrazone sodium salts are reported to yield cyclobu-tanones, which can be accounted for by rearrangement of an intermediate oxycarbene. In this manner, the sodium salts of dihydrofuran-2(37/)-one tosylhydrazones 1 were decomposed as a loose powder, at 310 C in a bulb-to-bulb distillation apparatus at an initial pressure of 0.1 Torr, to give the corresponding cyclobutanones 2 in addition to enol ethers, cyclopropanes and open-chain alkenes. Condensable products (74-76%) were collected at — 78 °C, weighed and the ratio of components was determined from their relative GC peak areas.63... 

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